Solar Shed

Chronicles of my efforts to install an automated solar powered air conditioner into our shed.

Current Goals:

Insulate

Obtain better performing 24VDC fans

Current configuration:

(7) 100W Mono panels

(7) 20A inline MC4 Diodes

(7) 10A inline MC4 Fuses

(1) 20A MPPT Solar charge controller

(1) 50A DC-to-DC charge controller w/MPPT

(1) 30A Mini-ANL fuse

(1) 80A Mini-ANL fuse

(1) 70A Mini-ANL fuse

(1) 150A Mini-ANL fuse

(1) 60A Bolt-on fuse

(2) 80A Bolt-on fuse

(1) 250A Termination block

(1) 1500W Inverter – pure sine

(2) 12V Deep cycle marine battery

(1) LiFePO4 50Ah battery

(1) 410W Window mounted air-conditioner

(1) Set 30A MC4 branch adapter 1:4

(1) Set 8AWG MC4 cables 15ft

(1) Set 10AWG MC4 cables 15ft

(2) Mini-ANL terminal blocks

(1) 4″ DIN Rail

(1) 25A DC DP-DT Circuit Breaker

(1) 32A DC DP-DT Circuit Breaker

4 AWG cable (black and red, about 8ft)

6 AWG cable (black and red, about 8ft)

8 AWG cable (black and red, about 12ft)

(1) MC4 Inline power meter 8AWG

(1) Inline power meter with shunt and display

(1) 12VDC Switchbox

Current Activity:

A Renogy DC-to-DC battery charger is really the heart of my system, it was a perfect way for me to add lithium to my already existing lead acid setup. However, I should not have assumed this unit would work with any run of the mill 100W panel (like pretty much any cheap PWM or MPPT controller might) because it has a very low voltage ceiling for an MPPT and will not charge from solar input if voltage is >25V. The voltage on my particular brand of panels (TP-Solar) is 27V ‘open circuit’. Adding insult to injury, if there are panels connected >25V anyway, the lead acid bank will not be used at all until solar is disconnected (draining the lithium with nothing at all to charge it or help with loads).

To work around this problem, I added a 20A inline MC4 blocking diode to each panel (when you add a diode inline voltage is reduced by .7V), but I still intermittently found it in the state where its not charging, so I added a higher current blocking diode (for 1.4V drop) just before the charge controller, and so far every day its charging as needed. This higher current diode generates quite a bit of heat so I have it mounted to a heat-sink and am trying to cool it with the 24v’ish input power from solar with tiny 24VDC fans, so they do not use any battery and only run when the sun is out.

Adding heat to my environment was not desirable with respect to my goals, and I suspect the whole problem can be addressed with firmware. All the unit needs to do is keep polling to see if still >25 and take action when this changes, manual intervention to disconnect/reconnect solar to eliminate >25 state should NOT be required 🙁

I have things set on a timer to cool the shed from NOON to 4PM and the cooler will only turn on with compressor @ 400W when the temp becomes >79F otherwise its just a fan that runs 80W which should be safe in any weather <79F, I can do 80W for 4 hrs in poor sun without draining the batteries.

I could go longer but, but I do not have to intervene with things set this way, and have it pretty much recharged for the night also. Shorter days may call for some adjustment, as things stand without insulation, best I can do is 6 degrees less than outside temp on 100F days, I could do much better.

Here is a rough sketch of how I wired the controllers:

Future Plans:

I have hot air rushing in from every crack, so I am planning to add caulking where this is obvious, and then reflective air bubble insulation over that. Might also need some stripping around door to make it seal.

(1) 500F Supercapacitor – This could help with voltage drop when compressor kicks in (rough on the equipment). Supercaps are known to greatly improve battery life, and last a very long time compared to batteries. Unfortunately they are cost prohibitive. To get the size needed to use my inverter with a constant load of 1500 watts I’d want 300 Farads which is not cheap. May be premature to invest in one of these now, but Im eager to experiment with these. I really hate lead acid, when my lead acid batteries are all used up I could see putting one of these in their place, and/or maybe even try a small 32V module for panel side of charger(s). Since MPPT chargers are ‘kind of’ a buck converter, they could be useful for draining a capacitor’s stored energy from high to low voltage, tapping their potential in a different way, although the only benefit that comes to mind doing something like that would be stabilizing power when a cloud passes.

(1) 600W Grid-tie plugin inverter – I am sometimes producing more solar than I am consuming, so I need to start throwing the excess at our electric bill. From what I understand these plug-in inverters are super easy, you just plug them into the wall to spin your meter backwards. Hard part is going to be figuring out the transfer mechanisms between what the off-grid needs, and what this thing gets. Grey as to whether this Amazon purchase would be legal to use, I suspect not. Whatever I end up buying must meet the following requirements based on my research thus far:

  • California Rule 21 compliant (supporting 2019 advanced features)
  • IEEE 1547 compliant
  • UL 1741 tested